A number of components you'll need for a BEAM robot are
built up from smaller parts;
many of these assemblies can be built up easily. In some
cases (solar engines), this is
the only way to procure the component; in other cases
(motor drivers, sensors),
"rolling your own" can save you money, at the cost of a bit
of your time.

In any event, these are some components you can easily
build:

Motor driversMotor drivers are
essentially small amplifiers, and turn a control signal
into a higher-current signal, suitable for driving a
larger motor. These are often integrated into a project's
circuitry, but I prefer to build them up as a discrete
component (this makes life easier if you "let the smoke
out").

Sensors
Most interesting BEAM projects require some sensors
in order to give the "critter" interesting interactive
behaviors. These range from simple contact sensors, to
acoustic proximity sensors, and generally must be built
up (there just aren't a lot of off-the-shelf robotic
sensors available).

Solar enginesSolar engines are used like
little power "savings accounts" -- solar power is stored
until it reaches a useable level, then the power is
released to drive a (cyclic) "burst" of robotic activity.
Often a solar engine is an integral part of a project;
you can also, though, build it up as a separate part, and
then attach it to your project (this makes it easier to
incrementally test your creation as you build it).

The "wobbler"...

Geared motorsMany BEAM projects require a DC motor
with a geared output (i.e., a motor with
attached gear train, so that the ultimate output
shaft rotates at about 40 - 50 RPM). I list these
as parts
since you can buy them directly (but they're not
generally cheap); you can save some money by
building one
up.